The invention broadly relates to a Raster Output Scanner (ROS) imaging system, and, more particularly, to a means and method for generating timing signals responsive to the detection of a scanning beam, at an image plane, crossing a fiber optic detector.
In conventional ROS systems, an intensity modulated light beam generated by a gas or diode laser is repetitively scanned across the surface of a photosensitive image plane to form a latent image of a document or the like represented by input binary data. Each scan line comprises composite images of individual pixels representing on and off states of the laser. These pixels must be aligned from scan to scan in the vertical or fast-scan direction; failure to do so results in the phenomenon known as "jitter". It is known in the prior art to position photodetectors at a start of scan line at a predetermined distance from the recording surface. Exemplary of the known detectors is a slit detector which compares the amplitude of a photodiode output signal against a predetermined fixed reference voltage. When the amplitude of the diode signal passes through this reference threshold, an indicator signal is generated. Also known is the so-called split detector which utilizes a two photodiode-dual comparator configuration to compensate for variations in beam output power. In operation, the sweep of the beam over the first detector sets an associated first comparator. The output of this first comparator is supplied as a reference for the second comparator, which is thereafter triggered by the sweep of the beam across the second diode detector to provide the indicator signal. An example of a split detector is disclosed in U.S. Pat. No. 4,386,272.
For many high speed, high resolution ROS systems, a solid state laser diode or a HeNe laser are the preferred mechanism for generating the recording beams. As is well known, the power output of these lasers varies in amplitude over time. The conventional slit detector when used with a laser scanning system is subjected to jitter since the signal generated by the detector will track, in amplitude, the gaussian shape of the beam as it sweeps across the face of the detector. The outputs produced by beams having different power levels will, necessarily, pass through the fixed reference level at different relative times (because of the photodide output rise time constant), resulting in the generation of indicator signals at different points in time in relation to the time reference base of the sweep of the beam. Since the synchronization of the scanner system is keyed to the time difference between the generation of the indicator signal and the transit time of the beam from the detector location to the targeted edge of the recording medium, this differential triggering effects a translation of information horizontally, or in the scanned direction, from line to line so that the picture elements do not align properly in the fast-scan direction. The split detector generates an output signal when the laser beam crosses between the two split detector sites. Since the time at which the beam crosses the two split detector sites is dependent on the split detectors physical spacing and not a fixed voltage reference, the output signal is stable (in time) independent of the beam power level. In other words, the split detector generates a signal which does not vary in time when the diode intensity varies. Both the slit detector and the split detector are configured in the same fashion; the detector is located on a remote detector board which is positioned adjacent the imaging surface. The light impinging on the detector is converted to an electrical signal which is then sent over wires back to a local electronics board which contains the circuitry to shape and process the detector signal and to generate the appropriate start pulse signals to the laser.
A third detection method is known in the art wherein a scanning laser position is sensed by placing a fiber optic bundle in the path of the scanning beam. The fiber optic bundle transmits the incident light to the local electronics board. The conveyed light energy is incident on an indicia as, for example, disclosed in U.S. Pat. No. 4,071,754, or on a photodetector, located on a local board. The detector converts the light energy into an electrical signal which is then processed to provide signals to the laser. Fiber optic detectors have several advantages over the split and slit detectors; they are less expensive, provide noise immunity and have simple mechanical mounting. Lower cost is realized because a separate scan detector circuit board is not needed and because the cable that provides power and signals to and from the remote scan detector board is not needed. Noise immunity is realized because the laser printer environment is electrically noisy (EMI, RFI) and the remote scan detector board has to get power and send the detected signal over wires through this noisy environment. The fiber optic system sends the signal as light energy through the noisy environment back to the local electronics board where the signal can be converted to an electronic signal. The local board would be in a controlled environment (shielded) where noise would be lower. Further, the mechanical mounting of the remote system is difficult because the scan detector board is relatively large. The fiber optic is very small and so is easier to locate and mount. Since the optical fiber can only transmit beam energy and not position, use with the split detector would be impossible and use with a conventional slit detector system would not be capable of generating a start of scan signal that does not vary in time when the scanning laser intensity changes.
The present invention is therefore directed to a means and method for generating a start of scan signal using fiber optic detection means which does not vary in time with scanning intensity changes of the laser diodes. The invention is based upon the insight of simulating the junction of the split detector by introducing a second, time delayed signal which corresponds to the second signal generated at the second photosite of a split detector. More particularly, the invention relates to a fiber optic scanning beam detector comprising fiber optic means positioned in the path of a periodically sweeping beam of light, said fiber optic means transmitting energy from said intercepted light onto a photosensor thereby causing said photo sensor to generate an electrical signal V1 corresponding to the intensity of said detected light, circuit means for dividing said electrical signal to form a second electrical signal, V2, said formation of said second electrical signal delayed in time from formation of said first electrical signal, and comparator means for comparing said first and second electrical signals and for generating a third electrical signal, V3, upon a detection of a crossover point between said first and second signal.